The objectives of this project are to determine the influence of microwave radiation on cardiac tissue using in vitro and in vivo methodologies. A method for exposing isolated rat atria to microwave radiation has been developed. The data suggest that 2.45 GHz CW microwave radiation of 2 or 10 mW/g has no overt effect on the rate of force of contraction of isolated atria. In addition, the response of atria to drugs was not influenced by microwave exposure. Specifically, the dose response curve for isoproterenol and acetylcholine was not influenced by either 1, 10 or 100 mW/g exposure and the ability of propranolol and atropine to inhibit the isoproteronol and acetylcholine response of rat atria was not altered by these exposure levels. Also, certain biochemical and physiological parameters, which are indicative of cardiac integrity, have been measured in unanesthetized rats during whole body ventral exposure to 2450 MHz CW microwaves. The data suggest microwave exposure of 10 mW/cm2 for 6 hr has no effect on mean arterial blood pressure of colonic temperature. However, there was a microwave induced bradycardia which was exhibited after 30 min of microwave exposure at 10 mW/cm2 and persisted throughout the remainder of the 6 hr exposure period. None of the biochemical or hematologic indices examined were influenced by this exposure level. Rats were also exposed to 2450 MHz CW microwaves for 6 hrs at 10 mW/cm2 in an ambient temperature of 27 degrees C. No change in heart rate was observed. When rats were exposed to 2450 MHz CW microwaves for 6 hrs at 10 mW/cm2 in an ambient temperature of 30 degrees C, tachycardia was measured.